Animal studies have provided a detailed analysis of muscle fiber types, muscle organization, neuroanatomical pathways and mechanisms of neural control of jaw muscles during such acts as mastication. The data from these studies have helped form the current explanations of the neurobiology and control of oral-motor function. However, recent data have provided evidence of a more complex anatomical and functional organization of the jaw muscles in animals but similar systematic studies have not been performed in human subjects. With the advent of new technologies, it is now possible to non-invasively image muscles to examine the detail of the anatomical organization and to simultaneously acquire multi-site recordings to analyze regional muscle activity or functional "compartments". It is imperative to understand the fundamentals of anatomical organization and functional control of normal human subjects before attempting to evaluate the abnormal muscular conditions which involve pain. Therefore, this proposal will examine the basic fundamentals of the anatomical substrate, functional organization and mechanisms of control of the human masseter muscle. A logical progression will be followed by first examining the masseter internal anatomical architecture to identify its structural partitions in cadavers and magnetic resonance images of humans, and then studying the functional neuromuscular compartments of the muscle by multi-site EMG recordings. This latter group of experiments include (1) mapping motor unit territory, recruitment order and motor unit characteristics; (2) investigating the effects of afferents on the recruitment and activity of motor units; and (3) comparing the inhibitory effects of cutaneous afferents from several intraoral sites on motor unit activity. After a thorough evaluation of the functional organization of the masseter muscle in normal subjects without pain, we will examine the same subjects with experimentally-induced facial pain and test a model that attempts to explain the changes in motor activity described in chronic pain patients.